1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a laser beam printer, or a facsimile.
2. Description of the Related Art
In a known image forming apparatus, such as a copying machine or a laser beam printer, when one color image is formed, a controller analyzes information from an external apparatus, e.g., a computer, and executes image expansion (transform) to image information per yellow, magenta, cyan and black. Further, an engine controls a printing operation of successively forming toner images on a photosensitive drum based on the image information obtained with the image expansion by using toners of yellow, magenta, cyan and black, and further executing multi-transfer of the toner images onto an intermediate transfer member.
A method for reducing a first print-out time (hereinafter referred to as “FPOT”) is proposed which executes the image expansion and the printing operation for each of the colors in parallel in the above-described type of image forming apparatus.
The term “FPOT” used in this specification implies a time from the start of operation of a printer until a first sheet is completely discharged onto a paper discharge tray. Also, the term “image expansion” implies a process of analyzing and expanding (transforming) a color image, which is to be printed, per color by the controller. Further, the term “printing operation” implies a process covering a step of forming a latent image per color on the photosensitive drum based on the image information that has been obtained by the controller executing the image expansion, a step of forming a toner image corresponding to each latent image, and a step of performing primary transfer of the toner image onto the intermediate transfer member.
However, when the above-described parallel processing method is employed and a long time is taken to execute the image expansion by the controller, there is a possibility that the image expansion is not completed until the timing at which the engine starts the printing operation. In that case, the engine cannot receive the image information and hence cannot execute the printing operation, thus resulting in a print error. To avoid such a print error, if the image expansion is not in time for the start of the printing operation, the controller transmits, to the engine, a signal for notifying the fact that the image expansion is not yet completed. Upon receiving the signal, the engine idly rotates a transfer drum once in a state where the printing operation is not executed, whereby the printing operation is postponed. A method capable of coping with the print error in such a manner is proposed in Japanese Patent Laid-Open No. 7-184019.
With the above-described related art, however, when the printing operation is postponed, at least a time corresponding to one rotation of the transfer drum is additionally required with the postponement of the printing operation. In other words, even when the image expansion is completed in the controller and the image information comes into a state capable of being transmitted immediately after the idle rotation of the transfer drum has been started to postpone the printing operation, the printing operation cannot be started until the transfer drum finishes one rotation. Thus, when the printing operation is postponed and the image expansion is completed in the controller during the idle rotation of the transfer drum, the time taken for the remaining idle rotation of the transfer drum becomes a loss time and the FPOT is prolonged.